The invention relates to a scanning device comprising a rotatable mirror for directing and aiming a radiation beam at a surface to be scanned, and a drive unit comprising a rotor section including a cylindrical, in particular disc-shaped, at least partly permanent-magnetic rotor body, carrying the mirror and supported so as to be rotatable about an axis of rotation, and a stationary stator section with coils extending into the magnet field of the rotor body, for generating electromagnetic driving forces acting on the rotor body to provide the rotary drive of the rotor section.
Such a scanning device is known from Japanese Kokai 61-147218. The known scanning device comprises a brushless electric motor having a stator with coreless flat coils. The electric motor has a rotor with two axially magnetized permanent magnets and mirror segments arranged at the circumferential surfaces of the magnets. The prior-art scanning device further comprises a centrally arranged mechanical bearing, in particular an air bearing, to support the rotor for rotation about an axis of rotation. The bearing has gaps between the bearing surfaces in which compressed air is introduced by means of a pumping device.
A drawback of the prior-art scanning device is the presence of the air bearing, in which such a turbulence can develop at high rotor speeds that an unstable bearing condition arises. However, for many uses of scanning devices comprising polygonal mirrors, high speeds and stable bearing arrangements are required.
A further drawback of the known scanning device is that in the course of time contamination in the bearing gives rise to wear of the bearing. Another drawback is that on account of the accuracy requirements imposed on it an air bearing is comparatively difficult to manufacture and is therefore comparatively expensive.